Many microelectronic devices are made of layered thin film structures with a plurality of interconnected functional layers that are conductive, semiconductive, insulating, doped, or protective. One example includes photovoltaic conversion devices, such as thin-film solar cells. Processes of manufacturing of such devices often involve multiple steps of laser drilling, cutting, scribing, and patterning of the layers of different materials including antireflection optical coatings. In particular, laser scribing is becoming a dominant technique for thin-film solar cell fabrication. However, an energetic laser beam can also inadvertently damage the underlying layer. It is difficult to recognize the point at which a laser action must be discontinued to ensure a complete removal of the desired layer(s), but at the same time prevent any damage to the underlying structure.
Fabrication of micro-electro-mechanical systems (MEMS) demands comparable processing of various layers and structures made of foils, semiconductors, ceramics, plastic masks, and metal stencils. In addition, many integrated circuits are encapsulated in plastic packages that have external contacts for electrically interfacing with the circuit. Similar plastic coatings are used on printed circuit boards. Sometimes such circuits fail. Commonly, manufacturers of such devices will analyze failed devices to understand the failure mechanism to determine whether a design change is warranted.
For conventional failure analysis of integrated circuits, a practitioner uses a wet etch to remove the plastic encapsulation. The etch is an acid that is selected to attack the plastic, but that is benign to the circuit. In this way, the plastic is removed while leaving the circuit with no further damage resulting from the plastic removal. Unfortunately, practitioners find that the wet etch technique is excessively time consuming and seek a more efficient plastic removal technique.
A laser pulse can attack and remove substantially any material on which it impinges. Because of the high temperature created by laser-material interaction, part of the ejected material will emit light that is characteristic of the ablated material. A laser can be scanned in a two-dimensional pattern across a surface to ablate the entire surface (e.g., the surface of a plastic integrated circuit package). During this process, conventional instrumentation cannot distinguish between the plastic and the underlying integrated circuit. Thus, a conventional scanning of a laser used to strip the plastic package or coating will either result in not exposing the integrated circuit or damaging the circuit, with neither such result providing the desired outcome.
Several diagnostics and metrology techniques are commonly used to characterize photovoltaic devices, including reflectometry, ellipsometry, x-ray fluorescence, x-ray reflectance, energy dispersive x-ray spectroscopy, secondary ion mass spectrometry, inductively coupled plasma mass spectroscopy, laser ablation inductively coupled plasma mass spectroscopy, scanning electron microscopy, transmission electron microscopy, scanning tunneling microscopy, atomic force microscopy, and other techniques. However, these established techniques are not practical for real-time diagnostic measurements during rapid laser processing of structured materials. X-ray techniques require long acquisition times. Mass spectrometry and electron microscopy require ultra-high vacuum. Most often used ellipsometry and reflectometry cannot measure opaque films and both are indirect techniques based on computational models with a large number of floating parameters. If the film properties deviate in such a way that the assumptions of the model are not valid, these techniques yield wrong results.